I haven't been in Mint 11 for a while so be prepared to rename the apps I talk about.
You open the Menu and choose an app called "Displays" (perhaps for you it's called "Monitors"?), choose the display you want to loose and Untick the "Same Image on all Monitors" (Perhaps called "Mirror Displays"). Then you should turn off one of the displays (probably want to switch off the Laptop one) and the other should be remain alone. Post back if you have any Issues/Questions.
Cheer up! Things are getting worse at a slower rate.